The present invention relates to a press shoe for a press of shoe type with extended pressure nip.
Known press shoes have hydrostatic pressure pockets the depth of which being substantially constant between their leading forward and trailing ends seen in the direction of rotation of the belt. Press shoes with this type of pressure pockets are shown, for instance, in U.S. Pat. No. 5,084,137 . With a pressure pocket shaped like this, the transition to the trailing land surface is very steep and this causes a number of problems. In order to cut out the pressure pocket the milling tool must be set in at least two different machining positions, thereby complicating manufacture of the press shoe and consequently increasing manufacture costs. During operation a hydrodynamic pressure is created within the region of the trailing land surface and the steep transition between the pressure pocket and this land surface induces the hydrodynamic pressure to increase very rapidly when it commences immediately after the pressure pocket. This in turn means that the paper web and press felts are subjected to relatively strong compression within a relatively short path of movement where this rapid increase in the hydrodynamic pressure occurs. Such a rapid compression of the press felts may at least briefly cause deterioration of their ability to absorb water from the web but, more important, desirable water flows are prevented from occurring in the web and the direction of the fibres as well as fibre density may be altered, with resultant deterioration in the quality of the paper. The problem is particularly serious at high speeds of the web and constitutes an obstacle to increase the speed. When the web is being threaded through the machine thicker sections are formed due to the web being inadvertently folded double. These thicker sections may also occur in the continuous web in the form of folds, collections of fibres or the like. Said thicker sections in the web will be displaced in the directions to the pressure pocket under the influence of the counter roll since the belt member is flexible, and they will then be affected via the belt member by the steep transition between the pressure pocket and trailing land surface. Immediately thereafter they reach the trailing land region where a very rapid increase in the hydrodynamic pressure occurs as mentioned above, with resultant compression of the web. The combination of influence from said steep transition and influence from a rapid compression within a short path of movement involves great risk of a breakage occurring in the web. Furthermore, the occurrence of thicker sections, particularly large thicker sections may damage the belt member. Also in this case the problem is particularly serious at high speeds.